Traditional liquid forging or squeeze casting produces metallic articles with a fine-grained microstructure by exerting a high pressure on at least partially molten metal during solidification. The articles formed have a high density and are ideally porosity-free. For example, on a punch die that moves into a female die to fully enclose a charge of at least partially molten metal, the at least partially molten metal solidifies into an article under external pressure continuously exerted by a hydraulic press on the punch die. Squeeze casting or liquid forging also reduces hot tearing or cracking in an article, effects which arise due to melt shrinkage during cooling. Such melt shrinkage can be compensated for by applying an oscillating squeeze pressure during solidification or by means of double acting pressure to improve article strength and toughness.
The actual volume of the article obtained depends on the quantity of at least partially molten metal supplied. To obtain articles having predetermined dimensions regardless of varying amounts of access of at least partially molten metal, a compensating pressure can be exerted through the cavity base. This compensating pressure allows excess material, which varies in volume from article to article, to extend beyond actual article dimensions. After ejection of the article, solidified excess material is trimmed off.
Current techniques are appropriately suited to the production of strong and tough articles having relatively simple shapes. However, more complex shapes require tighter dimensioning tolerances, which existing techniques are unable to achieve. For example, thinner sections tend to prematurely solidify, resulting in greater porosities in the thinner sections compared to the rest of the article, which leads to a non-homogenous structure. At the same time, current liquid forging or squeeze casting techniques as yet have no capability to form parts having relatively small through holes of fine tolerances. However, complex articles also often require homogeneously high strength and toughness, properties which are in themselves readily achieved by forging.
Also, when dealing with thin wall sections, quite often the grain size may be close to the same order of magnitude as the wall thickness. In that case a loss of one grain may cause a significant reduction in wall thickness and wall strength. For example, the grain size may be 100 microns and the wall may be 300 microns thick. This is important in small products such as, for example, the casings for small disk drives.